Heater

ABSTRACT

An elongated tube has a burner discharging products of combustion through its length. A layer of fluid to be heated is flowed over the outside of the firetube. Diverting structure is placed on the outside wall of the firetube to keep the flow rate of the fluid above a predetermined minimum and extend the surface of the firetube for heat dissipation into the fluids as a control of the film temperature of the fluid.

United States Patent Ferrin Sept. 17, 1974 [54] HEATER 1,880,533 10/1932 Thomasml 122/367 3,016,893 1/1962 Brown, Jr. 126/116 [75] Inventor: Charles Fem, Tulsa Okla 3,028,855 4/1962 Brown, Jr. 126 116 3 Assignee; Combustion Engineering Inc New 3,133,527 5/1964 Mizcr 122/136 York, NY. [2 F1 d M 2 1973 Primary Examinerl(enneth W. Sprague 1e ay Appl. No.: 356,692

2 U.S. Cl. 122/156, 122/367 C 51] Int. Cl. F22b 7/00 [58] Field of Search 122/136 R, 136 C, 155 C,

122/156, 367 R, 367 C; 126/116 R [56] References Cited UNITED STATES PATENTS 1,613,615 1/1927 Lippert .1 122/367 1,704,038 3/1929 Ellyson 122/367 ill I|| FLUID INLET 21 l E 8 4 t I N Attorney, Agent, or FirmArthur L. Wade [5 7 ABSTRACT An elongated tube has a burner discharging products of combustion through its length. A layer of fluid to be heated is flowed ouer the outside of the firetube. Di- ;erting structure is placed oil the ou tsfide uifill of the firetube to keep the flow rate of the fluid above a predetermined minimum and extend the surface of the firetube for heat dissipation into the fluids as a control of the film temperature of the fluid.

2 Claims, 3 Drawing Figures Pmmanwmw 3.835.816

SHEET 1 OF 2 i- HEATED FLU/D OUTLET FL U/D INLET HEATER BACKGROUND OF THE INVENTION 1. Field of the Invention I The present invention relates to heaters whose products of combustion are flowed into indirect heat exchange with fluid to be heated. More specifically, the invention relates to indirect heat exchange between products of combustion discharged through a cylinder and fluid to be heated directed through a chamber formed outside and concentric the cylinder. Structure within the concentric chamber is formed and arranged to provide the fluid to be heated with an area of heat exchange surface and a velocity which will prevent thermal breakdown of the fluid as it is heated.

2. Description of the Prior Art Direct fixed heaters are generally well known. The products of combustion, generated by burning various kinds of fuel, are flowed in contact with the surface of a container with fluid to be heated. The heat of the products is thereby transferred to the fluid in the container. Obviously, the fluid heated can, in turn, be used to heat other fluids in an arrangement which is described as the indirect heating of the second fluid.

The prior art has concerned itself with the use of both the radiant and convective heat from burners. Radiant heat, the direct heat of combustion, is difficult to control. For one thing, it is neither stable nor uniform. This results in the surface of the wall exposed to radiant heat having overheated areas. The fluid in contact with the other surface of the wall can, therefore, be overheated and thermally break down. The residue on the surface contacting the fluid will increase the unevenness of the heating pattern. Eventually, the wall of the surfaces can fail from this thermal stress. In general, the convective heat will have a more uniform pattern than this radiant heat.

Convective heat requires transport some distance of the products of combustion. The surface to be heated must be removed from sight of the combustion and be placed in contact with only the products of the combustion. This isolation of the surface to be heated requires the complication of ductwork and a separate heat exchanger structure to which the products are transported.

The most economical form for heater structure will be that which employs radiant heat. However, the prior art has found it difficult to economically provide the necessary safeguards against the dangers inherent in the use of radiant heat.

SUMMARY OF THE INVENTION A principal object of the invention is to simultaneously extend the area of effective heat exchange surface in the annulus between a heated cylinder and an outer shell and limit the lower rate of the flow of fluids to be heated through the annulus.

The invention contemplates a cylinder heated by the combustion and products of combustion of a burner discharging into one end of the cylinder. A jacket is arranged concentrically about the heated cylinder to provide an annulus as a fluid flow space. A structure is attached to the outside wall of the cylinder and extended into the flow space as a part of the effective area from which the heat of the cylinder wall flows into the annulus and at the same time establish a lower limit for the flow rate of the fluid through the annulus.

Other objects, advantages and features of this invention will become apparent to one skilled in the art upon study of the written matter, attached claims and drawing, wherein;

FIG. 1 is a sectioned front elevation of a vertical form of a heater, including the present invention;

FIG. 2 is a sectioned side elevation of a horizontal form of the heater; and

FIG. 3 is an end elevation of the heater of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring specifically to FIG. 1, there is shown a vertically extended structure which resembles a rocket. Legs 1 extend up from a base 2 at ground level to support a vertical, elongated body as though it were a rocket pointed at the moon.

Dramatics aside, the structure of FIG. 1 is a heater. Its appearance may be roughly that of a rocket, but it is actually a heater for fluids. Products of combustion are generated within this structure and flowed in indirect heat exchange with fluids requiring the heat of the products of combustion.

The fundamental building blocks of the heater is its firetube 3. This tube 3 is held in a vertical position. On its lower end 4, a burner is mounted in housing 5. Below the burner housing is mounted a flame arrestor 6.

This combustion equipment is conventional in form and function. Mounting the conventional burner to direct its flame upwards might be somewhat unusual. However, it is not necessary to disclose the details of how fuel is piped to to the burner, the air is controlled for combustion, and the firing rate is regulated. The important features of the invention are to be found in the arrangement between the firetube 3 and the structure which flows the fluids to be heated along the outside surface of the tube.

One of the principal forces which drove the design toward this rocket-like form is the small space on off shore platforms. A vertical, pencil form for a heater would be welcome on cramped oil well production platforms' With the firetube vertically extended, how could the fluid to be heated be efficiently flowed over the outside surface of the tube? The fluid to be heated must be flowed over the tube at a minimum velocity to keep a maximum allowable skin temperature. If the temperature of the fluid layer adjacent the tube surface goes above a certain range, the fluid will be broken down and components of the fluid deposited on the tube surface. The minimum velocity is tied to the composition of the fluid to be heated and the temperature rise necessary to meet the load on the heater. In practical sizes of heaters, it appeared that the thickness of the layer of fluid flowed over the tube surface would have to be in the order of 3/ l 6ths of an inch to keep the velocity of the fluid high.

It is simple to place a shell 7 concentrically about the outside of tube 3 and fluid-seal the shell to the tube at 8 and 9 to form annulus 10. But, to keep the two walls only 3/l6ths of an inch apart is not practical. Such a small space cannot be kept uniform. The fluid layer would be of uneven thickness. The entire operation would be erratic and unworkable.

What was needed was a structure mounted in the annulus to space the wall 7 from the outside wall of the tube. At the same time, the structure had to provide the fluid to be heated with a passageway which was uniform, yet small enough to keep the flow above the minimum rate which would avoid the elevated skin temperature which would lead to breakdown in the fluid itself.

The invention provides the required spacing structure. At the same time, while spacing and providing the required velocity, the structure embodying the invention extends the effective area from which the heat of the firetube flows into the annulus. This extended area for heat dissipation into the fluid limits the important skin temperature.

The inlet fluid conduit 11 is connected to the lower end of the annulus 10. Outlet fluid conduit 12 is connected to the upper end of the annulus 10. Fluid to be heated flows into the heater through inlet 11 and through the annulus 10 and structure in the annulus. The fluid, heated, flows from the heater through outlet fluid conduit 12. Conduit 13 then conducts the heated fluid to a point of use.

Thus far, the structure of the annulus has been considered more or less from a functional standpoint. More specifically, and structurally, we are looking at a fin 14 mounted on the outside surface of the firetube 3 in the form of a spiral. Shell 7 is spaced from the tube 3 by the height of this fin. When in place, shell 7 completes a spiral passageway which extends from inlet conduit 11 to outlet conduit 12. The spiral passageway completely covers the outside of tube 3 which is within the annulus 10.

Now it is not overlooked that the outer edge of fin 14 does not fluid-seal to the inside of shell 7. Not only is this beyond mechanical manufacturing skill normally available for this type of construction, but the varying thermal stress on tube 3, fin l4 and shell 4 opens gaps between sections of fin 14 and the inside of shell 4. However, by and large, the fluid to be heated will flow upward from inlet 11 in a spiral passageway with little leakage, or by-passing, from section to section.

To some extent, the disclosure to this point is couched in the broad, brush strokes of general principles. First, there is the thrust of the concept of the space-saving, vertical extension. Acceptance of this form leads to bathing the long, thin firetube with the liquid to be heated. Committed to this flow pattern, the danger of excessive skin temperature is met by both a high velocity for the fluid as it is heated and enlargement of the heat exchange area. The spiral fin then embodies the invention and provides a flow passageway with the proper dimensions to get the flow rate up high enough and the amount of heat exchange surface great enough to obviate a film temperature high enough to break down the fluid. This structure is shown clearly in FIG. 1 but the story is not satisfactory without some practical figures to give it meat and substance.

The first reduction to practice of the invention was a very practical heater. It was delivered to a location in Utah for heating the well-known Mobiltherm, a heat transfer fluid. The heated Mobiltherm was conducted into an oil well for a specific heating duty. At this particular location, the heater was required to deliver 1.5 MMBTU/hour. Around this requirement, and the limitation of the maximum diameter of firetube 3 on which a fin can presently be mounted, the design under the inventive concepts was developed.

Firetube 3 was limited to 18 inches in diameter which dictated about 21 feet of height to meet the heat demand. The wall was 1/2 inch thick and the material mild carbon steel. Fin 14 was 1V2 inches high and 1! 16th of an inch thick and welded in reaches spaced apart to form a passageway which would generate a 5-10 feet/second velocity of the Mobiltherm. With these dimensions, it was practical to form shell 7 of 3/8ths inch thickness about the finned firetube. All the problems of bringing the shell 7 to within 3/ l6ths of an inch of tube wall 3 were eliminated.

A Maxon burner was mounted in housing 5. The particular burner was of the inspirator type with a venturi of 2 inches. Combustion of fuel by this burner delivered the required heat to the firetube 3.

The temperature rise of 40 F of the Mobiltherm fluid, and the 5-l0 feet/second velocity of the fluid, kept its film temperature low, in the order of 50 F above the fluid temperature at any one point along the length of the heater. The test results indicated 52 percent gross efficiency, or 59 percent net efficiency. This is not an unusually high efficiency, but considering the simplicity of the construction and the acceptance of the arrangement of passing the fluid to be heated in a layer over the firetube, this was a surprisingly successful result. The conclusion is forced that a heater embodying these concepts had many applications on offshore petroleum production, as well as elsewhere.

The limits of conceptual thinking were not reached with the heater form of FIG. 1. FIGS. 2 and 3 disclose much of the invention embodied in a form which could be called horizontal.

Where the height becomes a severe limitation, FIG. 3 shows a firetube 20 could be provided with a U- section 21. Burner housing 22 and flame arrestor 23 are mounted on the lower end 24 of tube 20. Discharge of the products of combustion is then from upper end 25. Stack 26 is provided for end 25 to give an upward direction to discharge of the products.

Shell 27 is formed about tubes 20. Of course, the shell must be formed in sections. In FIGS. 2 and 3, the necessary flanges and bolts to enable this assembly to be formed about tube 20 are shown. The detail of this structure is readily apparent from the drawings.

Inlet 28 and outlet 29 connect into annulus 30 between tube 24 and shell 27. Fiber glass insulation 31 is placed over all the structure to reduce heat loss.

FIGS. 2 and 3, however, cannot utilize the annulus structure of a fin as can the FIG. 1 structure. At least, it would be very difficult to form the fin structure on U-section 21. However, there are other structures which can be used in annulus 30.

In FIG. 3, there is disclosed a material formed in solid bodies 32. These bodies 32 may take various forms and be of one of several materials. One thinks of ceramic as proper material. The bodies 32 may be nothing more complex than ceramic balls packed into annulus 30. The halls will have open spaces between them. Therefore, these open spaces will form a system of paths between the ends of the annulus which will control the rate of flow of fluid being heated. The diameter of the balls will fix the size of the fluid passages and, therefore, the rate of flow.

The same principles of the concepts developed with the FIG. 1 structure are found here, in FIG. 2. The

bodies 32 form the open spaces with a size to keep the flow rate of the fluid up. At the same time, the bodies 32 are in contact with the outside wall of the firetube 24 and with each other. Heat from the products of combustion within the firetube flows by conduction from the wall of the tube into the bodies 32 and from there into the fluid flowing between the bodies. Therefore, a grip is developed on the factors regulating the skin temperature. This structure securing this grip is quite simple, as disclosed in FIGS. 2 and 3. The structure is simple and economical to fabricate.

Although the horizontal space occupied by the FIG. 2 heater is inherently greater than that occupied by the FIG. 1 version, the other advantages are retained. The result is a heater which is gaining wide acceptability and one which attacks the heating problems of the customers while proving significant savings in construction over prior art heaters.

CONCLUSION The actual reduction to practice of this invention may be termed a firetube indirect heater of fluids. This term is to contrast the disclosed relation between the products of combustion, their firetube and the direct contact of the tube with the fluid to be heated with the relationships in the so-called cabin heater.

The cabin heater represents the relationship of the fluid to be heated passed in reaches of conduit arranged over internal walls of the cabin enclosure. The heat of products of combustion discharged into the cabin is absorbed by the conduit reaches and the exposed wall of the cabin enclosure. The walls absorb large amounts of the heat rather than the heat being directed into the fluid within the conduit.

Cabin walls must be given large amounts of expensive insulation to contain the heat of combustion. Alternatively, special arrangements of the conduit could be made to form what is now termed a water wall. However, this is very expensive. So, the cabin heater is faced with providing either expensive insulation for exposed walls or expensive arrangements for fluid conduit.

The firetube heater, on the other hand, avoids these cabin heater problems. The firetube contains the products of combustion and transfers their heat directly into the sheath, or layer, of fluid passed on the outside of the firetube. No refractory insulation is required. There is no wall to be protected. The tube wall, itself, becomes the water wall.

At the same time, the firetube heater does not have the danger from explosive mixtures collecting in it. The large volume of the cabin heater must not be allowed to fill with an explosive mixture. Any resulting explosion could tear the cabin heater apart.

In the flretube heater the explosive mixture does not offer a destructive threat. The volume of the tube is not great. The tube is generally able to withstand several hundreds of pounds of internal pressure without rupture. This means a great saving in safety features which the cabin type of heater must have to protect it.

From the foregoing, it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and inherent to the apparatus.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the invention.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted in an illustrative and not in a limiting sense.

I claim: In the claims:

1. A heater for fluids subject to thermal breakdown, including,

an elongated and cylindrical combustion chamber mounted in a vertical position,

a burner mounted on the lower end of the combustion chamber to direct flame and products of combustion up the length of the chamber,

a shell extended concentrically about the outside of the elongated combustion chamber to form an annulus about the combustion chamber,

an inlet fluid conduit connected to the lower end of the annulus for the introduction of fluid to be heated into the annulus,

an outlet fluid conduit connected to the upper end of the annulus,

and a means mounted in the annulus and on the outside of the chamber wall to extend the effective area from which heat flow into the annulus and sized to limit the cross-sectional area of the annulus to establish a lower limit for the rate of fluid flow through the annulus which will control the upper limit of the skin temperature at the outside of the chamber wall to prevent thermal breakdown of the fluid.

2. The heater of claim 1 in which,

the elongated combustion chamber is in the form of a tube,

and the structure mounted in the annulus is in the form of a tin mounted in a spiral on the outside wall of the tube to form a spiral passageway for the fluid and space the shell from the outside of the tube wall. 

1. A heater for fluids subject to thermal breakdown, including, an elongated and cylindrical combustion chamber mounted in a vertical position, a burner mounted on the lower end of the combustion chamber to direct flame and products of combustion up the length of the chamber, a shell extended concentrically about the outside of the elongated combustion chamber to form an annulus about the combustion chamber, an inlet fluid conduit connected to the lower end of the annulus for the introduction of fluid to be heated into the annulus, an outlet fluid conduit connected to the upper end of the annulus, and a means mounted in the annulus and on the outside of the chamber wall to extend the effective area from which heat flow into the annulus and sized to limit the cross-sectional area of the annulus to establish a lower limit for the rate of fluid flow through the annulus which will control the upper limit of the skin temperature at the outside of the chamber wall to prevent thermal breakdown of the fluid.
 2. The heater of claim 1 in which, the elongated combustion chamber is in the form of a tube, and the structure mounted in the annulus is in the form of a fin mounted in a spiral on the outside wall of the tube to form a spiral passageway for the fluid and space the shell from the outside of the tube wall. 